The present invention relates to a thermal head and a thermal head unit, which are used, for instance, in a miniature portable recording apparatus, a facsimile machine, a printer for tickets and receipts, etc.
A thermal head includes a head chip in which heat generating elements arrayed in a row and electrodes connected to these elements are provided on a ceramic substrate, and an IC chip serving as a driver for outputting print signals to selectively generate heat from desired heat generating elements at desired timings.
FIG. 9 shows an example of a thermal head unit in which the thermal head of this type is mounted onto a heat radiating plate to form a unit. The thermal head unit includes a thermal head 101, and a heat radiating plate 102 made of aluminum or the like. The thermal head 101 is designed such that an electrode 104 and a heat generating element 105 are formed on a ceramic substrate 103, and an IC chip 106 is further mounted thereon. The electrode 104, a separately provided external terminal 107 for inputting external signals therein, and the IC chip 106 are connected together through bonding wires 108. The IC chip 106 and the bonding wires 108 are molded with sealing resin 109.
Also, it known to provide a composite substrate using a ceramic substrate reduced in size. That is, as shown in FIG. 10, in place of the ceramic substrate 103, a ceramic circuit board 103A and a wiring substrate 103B such as a glass fabric based epoxy resin substrate (hereafter referred to as GE substrate when applicable) are used. In this case, the external terminal 107 is provided on the wiring substrate 103B.
A connection structure between the heat generating elements and the electrodes in the thermal head described above is classified into two types. The one type is a common electrode type in which a common electrode is provided at an end portion side where the heat generating elements on the ceramic substrate are arrayed. In this type, a segment electrode, which extends from a heat generating element corresponding to a print dot, is elongated to the other end portion of the ceramic substrate, and drawing wirings, which extends from both end portion of the common electrodes, are also elongated to the other end portion of the ceramic substrate. The other type is a so-called U-turn electrode type. That is, a pair of heat generating elements are provided correspondingly to a print dot, and one end portions of these heat generating elements are connected to each other through a U-shaped wiring. Further, one of the heat generating elements is connected to a segment electrode elongated to the end portion of the ceramic substrate, whereas the other one of the heat generating elements is connected to a common electrode provided at the end portion of the ceramic substrate. In either of the types, the common electrode are connected through external terminals, and the voltage is applied selectively to the respective segment electrodes through the IC chip.
In the thermal head of either of the types as described above, however, the common electrode is elongated in the array direction of the heat generating elements, and in general, both end portions of the common electrode are connected. Consequently, the electric resistance possessed by the common electrode causes variation in value of current flowing through the respective heat generating elements. That is, the value of current flowing through the heat generating element located at a central portion remote from the grounded portion of the common electrode is small to make the generated heat amount small, thereby causing variation in print density.
It is conceivable to make larger the width of the common electrode on the ceramic substrate to make the electric resistance of the common electrode small, thereby suppressing the print density variation. However, this is in contradiction to a demand of making the thermal head compact in size. That is, the ceramic substrate is made larger, and thus the entire thermal head is made larger.
Accordingly, in view of the problem described above, the present invention is intended to provide a thermal head and a thermal head unit, which can prevent print density variation while suppressing the size of the thermal head to be small.
A first aspect of the present invention relates to a thermal head comprising a head chip having one surface on which heat generating elements and individual segment electrodes and individual common electrodes connected to the heat generating elements are provided, and a semiconductor integrated circuit connected to the segment electrodes, the thermal head characterized in that an elongated common electrode connected to the individual common electrodes is provided on the head chip and is elongated in an array direction of the heat generating elements, and connections between the elongated common electrode and external terminals are provided at plural locations along the array direction.
A second aspect of the present invention relates to a thermal head, characterized in that the heat generating elements are arrayed on one end potion of the head chip, and the elongated common electrode is provided on the opposite end along the array direction of the heat generating elements.
A third aspect of the present invention relates to a thermal head, characterized in that a circuit board on which the semiconductor integrated circuit is mounted in joined to the head chip, and common electrode wiring patterns are provided to the circuit board for connecting the elongated common electrode to the external terminals.
A fourth aspect of the present invention relates to a thermal head according to the third aspect of the invention, characterized in that connection wires for connecting the common electrode to the common electrode wiring patterns are provided between physical blocks defined by the semiconductor integrated circuits.
A fifth aspect of the present invention relates to a thermal head according to the fourth aspect of the invention, characterized in that connection wires for connecting the common electrode to the common electrode wiring patterns are provided for the respective physical blocks defined by the semiconductor integrated circuits.
A sixth aspect of the present invention relates to a thermal head according to any one of the third to fifth aspects of the invention, characterized in that at least one connection wire for connecting the common electrode to the common electrode wiring pattern is provided within the physical block defined by the semiconductor integrated circuit.
A seventh aspect of the present invention relates to a thermal head according to any one of the third to sixth aspects of the invention, characterized in that each of the connection wires for connecting the common electrode to the common electrode wiring patterns is a bonding wire.
An eighth aspect of the present invention relates to a thermal head according to the seventh aspect of the invention, characterized in that at least part of the bonding wire extends across the semiconductor integrated circuit.
A ninth aspect of the present invention relates to a thermal head according to the seventh or eighth aspect of the invention, characterized in that at least part of the bonding wire is provided to extend through the semiconductor integrated circuit.
A tenth aspect of the present invention relates to a thermal head according to any one of the seventh to ninth aspects of the invention, characterized in that an end of at least part of the bonding wire is connected at a location between the semiconductor integrated circuits.
An eleventh aspect of the present invention relates to a thermal head according to any one of the third to sixth aspects of the invention, characterized in that each of the connection wires for connecting the common electrode to the common electrode wiring patterns is of a flip tip type.
A twelfth aspect of the present invention relates to a thermal head according to any one of the third to seventh aspects of the invention, characterized in that the semiconductor integrated circuit is of a flip tip type, which is mounted to extend across the head chip and the circuit board.
A thirteenth aspect of the present invention relates to a thermal head unit characterized in that the thermal head according to any one of the first to twelfth aspects of the invention is mounted to a support member.